If Paul had “not shrunk from announcing … all the counsel of God” (Acts 20: 27), how could he later claim that he had “done nothing against the ... customs of our forefathers” (Acts 28: 17)?
The questioner assumes that the expression “all the counsel of God” embraces the truth of the mystery, that is Gentile and Jew having equal status in one body on earth united to Christ the Head in heaven (as taught years later in Eph. 2: 14, 15; 3: 6). If that was so, then Paul could not be telling the truth in Acts 28: 17, for to teach that Christ had “broken down the middle wall of enclosure” (Eph. 2: 14) that separated Jew and Gentile was clearly “against … the customs of our forefathers” (Acts 28: 17). Again, in Acts 26, Paul claimed he had said “nothing else than those things which both the prophets and Moses have said…” (v22). Such claims cannot stand alongside the concept of the Assembly as the one body, for that truth sets aside the whole fabric of Judaism. However, Paul was telling the truth on both occasions. The error inherent in the question is not interpreting the words “all the counsel of God” in the context in which they are set.
In Acts 20: 28 Paul does speak of the Assembly, but as the flock, not the body. Now the figures of a flock and sheep are quite distinct from those of a body and its members. The concept of a flock allows more individual liberty than that of a body. Throughout Acts, Jew and Gentile were together in one flock as the Lord foretold in John 10, but were clearly not united in one body with no national distinctions. The Jewish brethren were called out of the fold of Judaism by Peter in Acts 2: 40 but still remained distinct from the Gentile brethren as the proceedings of Acts 15 clearly testify. While Paul was called to go to the Gentiles as well as the Jews (see Acts 9: 15; 22: 14, 15; 26: 16–18), that commission did not take him beyond the prophetic boundaries of the Kingdom given in the OT in which Jew and Gentile were together, but distinct (see Is. 60: 3; Mic. 4:1, 2; Zech. 8: 22). Such distinctions are maintained in the Kingdom but do not exist in the Assembly as the body of Christ. Thus the public revelation of the Assembly as the body of Christ was impossible while Israel remained publicly owned by God as His people (which was the case throughout the history given in Acts)—and what Paul taught at Ephesus concerning all the counsel of God was public teaching (see Acts 20: 20)!
Looking more closely at Acts 20, one might ask why Paul wanted to be in Jerusalem for Pentecost (v16)—a Jewish feast. He acted as a Jew because the nation had not been set aside, and the offer of the Kingdom to Israel in Acts 3: 17–21) had not yet been withdrawn. Yes, Paul had testified of “the glad tidings of the grace of God” (Acts 20: 24) but he had also “gone about preaching the kingdom [of God]” (v25), in keeping with the general testimony given throughout the book (see Acts 1: 3; 8: 12; 14: 22; 19: 8; 20: 25; 28: 23, 31). It is at this point in the account that he says “I am clean from the blood of all” basing that claim on the fact that he had “not shrunk from announcing to you all the counsel of God” (Acts 20: 26, 27). The words “preaching” (v25) and “announcing” (v27) are clearly connected, and so the expression “all the counsel of God” refers not to the Assembly, but to the Kingdom that he had just spoken of. Yes, Paul goes on to speak of the Assembly (see v28), but, as already noted, it is the Assembly as the flock not the body, and indeed, the language he uses in exhorting the Ephesian elders is language suited to the Kingdom. Thus he speaks of that which was able to give them “an inheritance among all the sanctified” (v32, my emphasis)—and the Kingdom is always presented on the line of inheritance (see Matt. 25: 34; 1 Cor. 6: 9, 10; 15: 50; Gal. 5: 21, Eph. 5: 5). The apostle speaks in similar terms later when he says that he was sent to the Gentiles “… that they may receive remission of sins and inheritance among them that are sanctified by faith in me (Acts 26: 18). While sanctification is also needed for the Assembly as the body of Christ (see Eph. 5: 26), the thought of inheritance is never used in relation to it.
Finally, let us turn to Ephesians to confirm what has been said. The first chapter leads up to “the Assembly, which is his body…” but we do not arrive there until the very last verse (v23). En route we pass through the truth of the Kingdom in its most exalted view (see vs 9–12). There the words counsel and inheritance (see v11) are both used in connection with the Kingdom, with Paul identifying himself with the believing remnant of Israel, speaking of them as those who had pre–trusted in the Christ before the complete nation is ultimately brought into submission prior to the Kingdom being established.
Hence I conclude that the words “all the counsel of God” in Acts 20 refer to the Kingdom, not the Assembly, and that Paul’s statements both to the Jews at Rome in Acts 28 and to Agrippa in Acts 26 were accurate.